This invention relates to infrared spectroscopy and in particular relates to FT-IR spectroscopy based upon a Michelson type interferometer.
In FT-IR spectroscopy an interferometer of the Michelson type splits an input light beam into a reflected beam and a transmitted beam by means of a beam splitter. Each split beam travels along its own path to a return mirror which deflects it back to the beam splitter along the same path. One of the return mirrors is stationary while the other is movable typically along a linear path between two limits equidistant from a datum position. At the beam splitter the return split beams recombine along a common output path leading to a photodetector via a sample station.
If the movable mirror is at its datum position the optical path of the two split beams is the same so that when those split beams return to the beam splitter they constructively interfere. This results in a large signal being produced at the photodetector and this is known as the centreburst. If the movable mirror is shifted towards the incoming split beam, the optical path of that beam decreases and conversely if it is moved away the optical path is increased. Thus, as the movable mirror is moved from one limit to another two complete series of optical path difference values of opposite signs are generated and this travel is referred to as an optical path difference (OPD) scan. The output signal of the photodetector during an OPD scan is a series of superposed electrical sine waves of different frequencies and amplitudes. This signal is known as an interferogram.
These interferometers also include a reference light source, typically a laser, which is used to measure the optical path difference. The reference fringes created during an OPD scan are sensed by a photodetector which generates a reference fringe signal which is a sine wave.
When no sample is present at the sample position the detector signal is the emission interferogram of the light source typically an infrared source. When a sample is present the output signal of the detector is the interferogram of the sample superimposed upon that of the light source. By taking the Fourier Transform of the source interferogram and the Fourier Transform of the sample interferogram superposed upon that of the source it is possible to obtain the spectrum of the sample.
In modern interferometers the interferograms are acquired and processed digitally in order to obtain the spectrum of a sample under investigation. It is known, for example, to feed the output of the photodetector to an analog-to-digital converter in order to produce a digital representation of the interferogram. The reference fringes from the laser can also be digitised in a similar way. One way in which this can be implemented is described in our co-pending European Patent Application No 96307360.6 (E-PA-0836083).
As referred to above the reference laser is used to determine changes in the optical path difference of the interferometer and hence to determine the optical path difference interval at which the interferogram is sampled by the analog-to-digital converter. The interferometer sinusoidally amplitude modulates the laser beam and one period of the sine wave corresponds to a change in optical path difference equal to the laser wavelength.
In carrying out an analysis of a sample an interferometer will execute a number of scans sweeping forward and backwards through the centreburst of the infrared interferogram generating a series of ADC readings during part of each scan. The length of each scan is determined by the required spectral resolution. The reference fringe is used to determine the exact times at which the data converter in the interferogram channel should be read in order to build up a sampled interferogram with constant optical path difference intervals. In practice changes in scan direction may occur at slightly different optical path difference values in different sweeps. Lack of detailed knowledge about where reversals occur in the fringe waveform will lead to some uncertainty in the absolute optical path difference of points read by the analog-to-digital converter in subsequent scans. In an ideal arrangement the exact OPD at which each data point is read should be known in each sequence of readings (i.e. each scan) and this is essential where interferograms are being co-added. A number of methods have been used in order to achieve this. If this is not done the position of the optical path difference could vary by perhaps a few microns between each scan and this can significantly affect the accuracy of co-added interferograms and consequently affect the quality of the transformed spectra.
One known way of achieving this requirement is to use a correlation algorithm. The first completed interferogram is centred around the maximum value of the data which is assumed to be the centreburst and subsequent completed scans are correlated against this in order to detect any data shift which gives the best correlation with the first (or accumulated) scan. This technique only works if the infrared interferogram has a reasonable signal-to-noise ratio and can fail with certain samples such as narrow pass band optical filters.
Another way of achieving the requirement is to use a method based on absolute counting. This involves continuous up/down counting of laser fringes with the optical path difference known exactly at all times after an initial calibration. The data acquisition is started on the same count value and hence the same OPD value for each sweep. A critical factor in this approach is the determination of the instant at which the scan mechanism changes direction and hence the need for a change in the direction of counting.
One known way of implementing absolute counting is to use an up/down counting system based on fringe quadrature. This involves providing extra optical components and circuitry to generate two reference fringes nominally 90xc2x0 apart. Other known ways are to ensure that the reversal in direction occurs at a particular phase of the laser fringe, or to use phase modulation of the OPD drive.
The present invention is based upon the concept of representing the amplitude of the references fringes by a plurality of amplitude states and identifying a reversal in scan direction by the occurrence of a particular sequence or sequences of such states. One example in accordance with the invention uses an arrangement similar to the type described in the above-mentioned European Patent Application No 96307360.6 in which the reference fringe is sampled by an analog-to-digital converter at constant time intervals from which are determined the zero crossing points of the laser fringes, that is the time at which the fringe signal changes from a positive to a negative value or vice versa. From this fringe counting can be achieved. The present invention involves a technique for analysing the reference fringe data to identify characteristics which indicate the position of reversals of direction, and hence to provide the ability to achieve absolute up/down counting.
According to the present invention there is provided apparatus for processing the output signals of a Michelson type interferometer used in Fourier Transform spectroscopy which outputs include a waveform comprising an interferogram and a reference waveform representing interference fringes, said apparatus includes means for providing a digital representation of the interferogram waveform and means for providing a digital representation of the reference waveform and processing means for processing the digital representation of the reference waveform which processing includes monitoring parameters of the reference waveform signal and recognising a reversal of the scan direction by a change in said parameters. The monitoring may comprise monitoring the amplitude of each half fringe and a reversal is recognised by a failure of the fringe signal during a half cycle to attain a given percentage of the amplitude of preceding fringes. The monitoring may comprise monitoring the time occurrences of each half cycle and a reversal is recognised by a change in the interval between successive half ranges. In a preferred arrangement the reversal of scan direction is recognised by identifying either a failure of the fringe signal during a half cycle to obtain a given percentage of the amplitude of preceding fringes, or by recognising a change in the interval between successive half fringes. The digital representation of the reference fringe waveform may be generated by analog-to-digital converter sampled at constant time intervals. The digital signal processor can carry out fringe counting by counting said zero crossings of the reference waveform and change count direction when a reversal in scan direction is detected.
An alternative implementation uses a zero crossing detector and analog comparators to determine whether the fringe amplitude exceeds a set percentage of the peak fringe value during the previous half fringe of the reference waveform, and a microprocessor for timing analysis and counting.
Another aspect of the present invention provides a method of processing the output signals of a Michelson type interferometer which outputs include an interferogram and a reference waveform representing interference fringes, said method comprising providing a digital representation of the reference waveform and processing the digital representation using a digital signal processor said processing including monitoring selected parameters of the reference waveform and recognising a reversal of the scan direction by a change of said parameters.